Chapter 2.3 Dronning Maud Land Jurassic volcanism: volcanology and petrology

Abstract The Jurassic igneous rocks of Dronning Maud Land represent Karoo flood basalt magmatism in Antarctica. Fifty years of research has documented systematic differences between magmas associated with the Karoo rift-zone (Vestfjella and Ahlmannryggen) and the rift-shoulder (Sembberget, Kirwanveggen) settings. The 189–182 Ma rift-zone tholeiites were chemically diverse and mainly formed compound-braided flow fields which record several magnetic polarity reversals. In contrast, the c. 181 Ma rift-shoulder tholeiites were chemically uniform and formed thick tabular sheet lavas within a single normal polarity period. The volcanic architecture! records a long initial phase of slow eruptions from shield volcanoes in the initial rift and a brief phase of voluminous fissure eruptions flooding the rift shoulder. All of the major magma types in the rift-zone and rift-shoulder settings belong to a Nb-depleted category of Karoo flood basalts and were mainly derived from depleted convective upper mantle by magmatic differentiation. Pyroxenite-rich mantle components may have been significant sources for the most enriched magma types. Geochemical fingerprints of recycled crustal material imply that the Nb-depleted Karoo tholeiites may have been derived from mildly subduction-modified parts of the same overall upper-mantle reservoir which has been associated with the Ferrar tholeiites.